Presently all structural polymers that are manufactured contain one or more compounds derived from petroleum sources. Because of this, the cost to produce these polymers will fluctuate with the cost of oil. The recent volatility in the oil market has resulted in a substantial increase in the cost per barrel with a corresponding increase in the chemicals and compounds obtained from petroleum. These price increases are compounded by the fact that oil reserves are finite and non-renewable.
In addition to rising manufacturing costs, health issues arise with these polymers because some of the compounds used in their production are known endocrine disruptors. Endocrine disruptors interfere with the body's endocrine system and produce adverse developmental, reproductive, neurological and immune effects in humans. Some of the commonly used chemicals used in the production of plastics suspected of acting as endocrine disruptors include diphenyl ethers, bisphenols, styrenes and a variety of phthalates. Exposure can result from direct contact with these chemicals or through ingestion of contaminated, food, water or air. Industrial workers can be exposed to these chemicals through direct contact when working with resins or plasticizers that contain these compounds. In addition, byproducts of some chemical and manufacturing processes including burning of plastics can release endocrine disruptors into the air or water.
Recent studies indicate that ingestion of endocrine disruptors can result when these chemicals leach out of plastics used in everyday products such as plastic food containers and plastic water bottles. In addition, many endocrine disruptors are persistent in the environment and accumulate in fat so the greatest exposure can come from consuming fatty foods and fish from contaminated water. While a variety of pathologies may eventually be linked to these compounds, current investigations indicate that endocrine disruptors may contribute to miscarriages, lower fertility, increased incidence of endometriosis, obesity and some cancers.
Styrene, which has been identified as an endocrine disrupter has also been classified as a possible human carcinogen by the World Health Organization's International Agency for Research (IARC), Environmental Protection Agency (EPA) and The National Institute for Occupational Safety and Health (NIOSH). Styrene is a significant component in many current structural formulations wherein it is used as a significant monomer with or without bio-based materials. The EPA OPPT Chemical Facts Sheet for styrene eludes to this fact and states that “(T)he consumption of styrene in the United States may further decline in the future due to the Clean Air Act mandate on reduction in the volume of allowable styrene emissions”.
Like styrene divinylbenzene is also identified as an endocrine disruptor and is a significant component in structural polymer formulations. It is used with or without bio-based materials to impart cross-linking and rigidity. The concern for this compound is suggested in the divinylbenzene Material Safety Data Sheet (MSDS) which states “the toxicity of divinylbenzene may resemble that of styrene” and NIOSH which states “Insufficient data are available on the effect of this substance on human health, therefore the utmost care must be taken”.
Because of these and other environmental reasons, alternative compounds and sources of compounds that eliminate these concerns have now become the focus of global research. Materials derived from renewable resources such as plants offer an abundant, low cost and sometimes biodegradable alternative to their petroleum-based predecessors (Andjelkovic, D. D. et al., Journal of Polymer, 46:9674-9685 (2005)). To date, a variety of natural occurring plant oils have been utilized to prepare polymers (Wool, R. P. and Xinzhi, S. S. “Bio-based polymers and composites” Amsterdam; Boston: Elsevier Academic Press (2005) and Wool, R. P. and Shrikant, N. K. “Bio-Based resins and natural fiber”. ASM International Materials Park, Ohio, USA (2006).
Natural plant oils such as soybean and linseed consist predominantly of triglycerides which are esters comprising a glycerol moiety and three molecules of fatty acids. Of the known fatty acids that comprise triglycerides the highly unsaturated fatty acids such as linoleic and linolenic are of particular interest for the preparation of polymers. It is the unsaturated carbon-carbon double bonds of the fatty acids that are utilized in polymer formation. The degree of unsaturation is represented by the iodine value of the triglyceride and is the amount of iodine (mg) that will react with a 100 gram sample under specified conditions (F. S. Güner et al Prog. Polym. Sci. 31:633-670 (2006)). Triglycerides with iodine values greater that 130 are considered drying oils. These oils are the most widely used in the coatings industry in the preparation of oxypolymerized oils, polyesters, polyesteramides, polyamides, polyurethanes, acrylic resins and epoxy resins.
Formulations containing bio-based oils such as triglycerides have been used historically for a variety of purposes. These formulations often comprise acrylated epoxidized triglyceride resins and are generally flexible minimally cross-linked amorphous polymers with little or no structural strength. They are often used in surface coatings including varnishes, lacquers, caulking, adhesives, printing inks and shoe polishes. The flexible chains of triglycerides have been considered valuable for low modulus materials but not for use alone in preparing high modulus materials.
Crosslink density has been recognized as an indicator of the rigidness of a polymer. Flexible polymers typically have low crosslink densities while high crosslink densities have been recognized as necessary for high modulus materials. Rigid aromatic cross-linking reagents such as divinylbenzene and bisphenol A have been used to create higher modulus polymers with triglycerides, sometimes using styrene to enhance rigidity (Lewis U.S. Pat. No. 4,040,994, Drzal Macromol Mater. Eng. 289; 629-635, (2004), Larock Biomolecules 6:797-806, (2005)). These types of polymers have been developed from modified soybean and linseed oils (Warth H. et al., Die Angewandte Makromolekular Chemie., 249:79-92 (1997) at the University of Delaware, Iowa State University, and the Michigan State University.
Richard Wool at the University of Delaware focused on bio-based materials produced from different triglyceride monomers utilizing different amounts of styrene (ST). Acrylated epoxidized soybean oil (AESO) monomer was prepared by reaction of acrylic acid with epoxidized triglycerides. Subsequently, AESO was blended with reactive diluents, such as styrene to improve the processability of AESO and to control the resultant polymer properties in order to achieve a wide range of structural applications. The pure AESO polymer exhibited a tensile modulus of 440 MPa, with a tensile strength of 6 MPa. When the AESO polymer was blended with 40% of styrene, the moduli was 1.6 GPa with a 21 MPa tensile strength.
Maleinized soybean oil monoglyceride (SOMG/MA) was synthesized by a two step process from triglyceride oil. A standard glycerolysis reaction was performed converting the triglyceride to monoglyceride followed by the addition of 2-methylimidazole, ST and triphenyl antimony with a 3:2 weight ratio of glycerol to maleic anhydride. The copolymer of SOMG/MA with styrene had an observed tensile strength equal to 29.4 MPa with a modulus of 0.84 Gpa. The addition of diols such as neopentyl glycol (NPG), and bisphenol A (BPA) during maleinization increased the tensile strength to 15.6 MPa with a modulus of 1.49 GPa.
Acrylated epoxidized linseed oil (AELO) was synthesized from epoxidized triglyceride by treatment with by acrylic acid (AA), and the residue epoxy groups and the newly formed hydroxyl groups were subsequently modified using maleic anhydride in the presence of ST. AELO formed rigid polymers when polymerized with comonomers such as styrene.
Richard Larock at the Iowa State University developed a number of chemistries to convert triglycerides into useable materials. Among these are cationic, thermal, and free radical polymerization Mohanty, A. K. et al., “Natural Fibers, Biopolymers, and Biocompositions” Chapter 23. CRC Press, Taylor & Francis Group, Boca Raton (2005).
Cationic copolymerization of soybean oil (SOY), ST, and divinylbenzene (DVB) gave dense polymeric materials that ranged from elastomers to rigid plastics. When thermal copolymerization of tung oil, ST, and DVB was performed a, transparent, rubbery material was obtained. Radical polymerization was performed in order to copolymerize conjugated vegetable oils with acrylonitrile. The resultant polymeric material was transparent and rigid (Mohanty, A. K. et al., supra).
A series of bio-based, shape memory polymers were also synthesized by cationic copolymerization of soy oil with cross-linking agents such as dicyclopentadiene (DCP, a by-product of petroleum) initiated by boron trifluoride diethyl etherate (BFE) or modified using Norway fish oil (NFO), or fatty acid ester, which resulted in a series of new thermosetting rubbery materials with variable degree of rigidity (Mohanty, A. K. et al., supra; Larock, C. R. and Andjelkovic, D. D., Polymeric Materials, 93:882 (2005), Larock, R. C. and Fengkui L., Journal of Applied Polymer Science. 84:1533-1543 (2002), Marlen, V. and Larock, C. R. (2005), Polymer, 93:767 (2005) and Larock, H. P. and Richard C., Polymeric Material, 93:768 (2005)).
As shown in the table below, a variety of polymeric materials ranging from elastomers to rigid plastics were synthesized from the cationic polymerization of various soybean oils and petroleum-based alkene monomers.
CrosslinkdensityDampingPolymerTg (° C.)(mol/m3)(tan δ)SOY45-ST07-DVB40-[NFO5-BFE3]864.8 × 1030.09SOY45-ST00-DVB47-[NFO5-BFE3]725.7 × 1030.07LSS45-ST00-DVB47-[NFO5-BFE3]711.6 × 1040.19CLS45-ST00-DVB47-[NFO5-BFE3]1054.0 × 1040.10
Lawrence Drzal at the Michigan State University has prepared bio-based materials consisting of ELO with petroleum-derived additives in large proportions including diglycidyl ether of bisphenol F (DGEBF), methyltetrahydrophthalic anhydride (MTHPA), used as a curing agent and 1-methylimidazole used as an accelerator (Miyagawa, H, et at., Macromol. Mater. Eng., 289:629-635 (2004)). It was observed that increasing the amount of ELO in the polymer decreased the storage modulus from 3.2 to 1.7 GPa. Thus, it was possible to replace a large amount of petroleum-based DGEBF with ELO with deleterious effects on storage modulus.
In all of the research noted above and as was generally understood at the time of the investigations, aromatic petroleum-based compounds were required and used as agents to increase rigidity of the plastic obtained from bio-based oils. Recently, a number of these petroleum based compounds have been found to act as endocrine disruptors. Leaching of these compounds from polymers containing these compounds used in food storage or as liquid containers has raised significant health issues. Additional concerns arise for workers during formulation and processing of these polymers when toxicological hazards are present.
Consequently, there is a need in the art for structural plastics that do not utilize petroleum based compounds and are prepared solely from renewable source materials such as unsaturated triglycerides from plant oils with renewable non-petroleum-based comonomers.